Grounding the secondary of a step-up 3 phase transformer

[ichudov]

Today, I set myself up with a three phase, 45 kVA, step up transformer, converting my 240v to 480v. My building is 240v three phase and I need 460 volts to test various 460v equipment.

This transformer really is a step down transformer, that I hooked up backwards. The "high side" is delta connected, and I do not see any way to change that.

Picture is here:

https://picasaweb.google.com/100231737176390367041/DropBox?authkey=Gv1sRgCIDl5a7JuZq35gE#5693510756662101026

The transformer seems to be working fine, however, I have a grounding question. Given that this is 460v delta, what, if anything, should I connect to ground? Would it be proper to ground one of the legs of the 460v three phase secondary, to ground?

Thanks

i

 

[templdl]

It is impossible to provide you with an answer give that you gave us no information about the transformer in question.
You said that you supply was 240v which one would assume is a delta. But with out information regarding the transformer that you intend to use to answer your question would be based on only an assumption which may be the wrong answer.
Then, please advise the nameplate information off of the transformer. Give this information it would be a piece of cake for any of us to answer your question quickly and accurately,

 

[ichudov]

templdl, I will do so tomorrow. Thank you. I was a little afraid to post too many pictures, as a new member.

 

[ActionDave]

The transformer will work fine, but what happens if there is a ground fault on the secondary is an issue.

What kind of equipment are you testing that requires 480V? Motors?

 

[ichudov]

The transformer will work fine, but what happens if there is a ground fault on the secondary is an issue.

What kind of equipment are you testing that requires 480V? Motors?

Yes, mostly machines with motors, such as hoists, lathes, and so on.

i

 

[Marvin_Hamon]

With the 3W 480V your grounding choices are rather limited. You can corner ground the delta, but many folks are not that happy with that choice. I know I don't like the idea that there is a 480V to ground potential hanging around to bite me. You could add another transformer as a grounding transformer which would give you a derived neutral that you could then ground, then you would have only 277V to ground ready to bite you. You could also just add a ground detector to the 480V side to warn you that one of the legs has become grounded and needs to be repaired. I would take door number 3 for myself.

 

[jim dungar]

With the 3W 480V your grounding choices are rather limited. You can corner ground the delta, but many folks are not that happy with that choice. I know I don't like the idea that there is a 480V to ground potential hanging around to bite me. You could add another transformer as a grounding transformer which would give you a derived neutral that you could then ground, then you would have only 277V to ground ready to bite you. You could also just add a ground detector to the 480V side to warn you that one of the legs has become grounded and needs to be repaired. I would take door number 3 for myself.

You don't like a 480V to solid ground voltage, but you are okay with one of from 277V to 960V depending on if faults are present.

If this is feeding a single device/machine/plug then I would normally go ungrounded, like Mr. Hamon, as the possibility of a faulted conductor being left unresolved is pretty low. Actually ground fault tripping would also be an option.

However if this is feeding several pieces of equipment or a panel, then I usually recommend a corner-grounded system.

 

[ichudov]

You don't like a 480V to solid ground voltage, but you are okay with one of from 277V to 960V depending on if faults are present.

If this is feeding a single device/machine/plug then I would normally go ungrounded, like Mr. Hamon, as the possibility of a faulted conductor being left unresolved is pretty low. Actually ground fault tripping would also be an option.

However if this is feeding several pieces of equipment or a panel, then I usually recommend a corner-grounded system.

It appears to me that, if I leave the 480v delta ungrounded, then, if one phase accidentally energizes the body of a machine that I woould be testing, then still I would avoid being shocked.

Re: one or many machines on this circuit. The answer is that this is for testing only, and I do not have any plans or provisions to connect more than one machine at a time.
i

 

[ichudov]

Here's a picture of the transformer data plate.

http://yabe.chudov.com/tmp/Transformer-Data-Plate.jpg

 

[Joethemechanic]

I'm liking the corner ground myself. Although just for testing purposes I might not ground the 480 delta at all. Although I would make some kind of provisions to have the machine at the same potential as the ground that I am standing on.

Is this thing going to be operated by only you? Or might employees be using it?

 

[jim dungar]

It appears to me that, if I leave the 480v delta ungrounded, then, if one phase accidentally energizes the body of a machine that I woould be testing, then still I would avoid being shocked.

Sorry, but this is not true. If one phase goes to ground, you now have an unintentional corner grounded system.

 

[ichudov]

I'm liking the corner ground myself. Although just for testing purposes I might not ground the 480 delta at all. Although I would make some kind of provisions to have the machine at the same potential as the ground that I am standing on.

Is this thing going to be operated by only you? Or might employees be using it?

Yes, I agree that I will be grounding bodies of machines. What I wanted to say, rather, is that if I have ungrounded 480v, then if my body makes accidental contact with one phase, I may avoid getting shocked.

I do indeed have full grounding path and all provisions for grounding bodies of the machines being tested.

I do not plan on letting anyone else use it.

 

[infinity]

Here's a picture of the transformer data plate.

http://yabe.chudov.com/tmp/Transformer-Data-Plate.jpg

This is a standard Delta/WYE transformer that you plan on reverse wiring? (photo) If so, your choices on the secondary are ungrounded, which would require some sort of ground detection device or corner grounded, by grounding one phase.

 

[jim dungar]

... I may avoid getting shocked.

You will not get shocked if you are wearing the proper PPE.

A truly ungrounded system does not exist. If you contact (1) live conductor + ground, you will be in parallel with the capacitive coupling of your system and some current could flow through you.

 

[templdl]

Most awesome infinity.

Bring L1, L2, and L3 from your source along with an EGC. Connect to X1, X2, and X3 and attach the EGC to the ground lug of the transformer.
On the load side you have H1, H2, and H2 which would be a delta. Bring each one out as your L1, L2, and L3. Then choose H1, or H2, or H3 and ground it turning it into a grounded 'B' phase attaching the grounding phase to an EGC which is brought along with the phase conductors.

It is important to ground one of the phases in order to provide a path back to the source should one of the (2) ungrounded phases go to ground. As such it would allow the OCPD to trip should this occur.
Remember that there will be no neutral as you will only have a 3ph3w system.
If you leave it ungrounded should one phase go to ground nothing will happen other than the chance for capacitive coupling as Jim indicated. But all hell will break loose should a second phase go to ground as the current path between the points where the 2 phases are grounded will be unpredictable and as such very dangerous. The current path could be anywhere across a facility.

There are applications where it is desirable to have an ungrounded system but ground detector systems and monitoring devices are added to indicate if a phase goes to ground to allow a qualified person to identify the grounded point before a second conductor goes to ground.

In addition you taps will be on the secondary which can be a bit confusing at first should you need to use them.

 

[ichudov]

templdl, thanks.

So, it would appear, then, that it would be advisable to ground one corner, and it is safer than leaving it ungrounded?

This, sort of, is a judgment call, as to how likely it is to have fault in one phase only, or two phases, and what the hypothetical fault current path would be.

As I mentioned, my intended use of this circuit would be to test 460v devices that I buy and sell. The typical use would be to hook up the transformer, power up the device, run it to see if it works, maybe shoot a video, and then disconnect everything. The device to be tested would likely be sitting on the floor, not hooked up to, say, compressed air lines or water lines.

Regarding taps: yes, since I hooked the transformer backwards, the taps are on the secondary (high side).

What is worse, is that, since it is a 480 -> 208 transformer, none of the existing taps would get me high voltage under 460v. The lowest I would get is 511 volts.

I had to do a little bit of math with a calculator and to pull one winding a little away, and tap into it, with a split bolt. This gives me actual voltage of 476 volts when the transformer is fed with 240 volts, which is acceptable. In other words, I sort of created my own tap.

 

[Marvin_Hamon]

You don't like a 480V to solid ground voltage, but you are okay with one of from 277V to 960V depending on if faults are present.

If this is feeding a single device/machine/plug then I would normally go ungrounded, like Mr. Hamon, as the possibility of a faulted conductor being left unresolved is pretty low. Actually ground fault tripping would also be an option.

However if this is feeding several pieces of equipment or a panel, then I usually recommend a corner-grounded system.

I would be interested in hearing what fault would give me 960V to ground using a derived neutral.

Thanks

 

[templdl]

ichudov,
You are going to have to determine if an ungrounded delta is advantageous in your application. If you are not being able to identify a legitimate justification to do so a corner grounded delta makes more sense.
As Jim related, even though it may not make any sense, the voltage of an arcing fault in an ungrounded delta can far exceed the actual L-L voltage I believe because of capacitance if I remember correctly.

Instead of considering the technicalities of an ungrounded delta it is important import to first have an application reason as to why it is reqired and then take into concideration the safety issue in doing so.
It is not uncommon someone to be reluctant to intentionally ground a phase conductor of a delta because to them it doesn't make sense.

 

[Joethemechanic]

I would be interested in hearing what fault would give me 960V to ground using a derived neutral.

Thanks

I am thinking this would be some kind of primary to secondary fault/faults creatind some kind of autotransformer, I was going to do the math, but I was too lazy. If you wanna do the math, don't forget to put the phase shift between wye and delta in the calculations

 

[Marvin_Hamon]

For a simple test setup I am assuming you will not have any overcurrent protection on the secondary side, maybe not even a disconnect on the secondary side if you disconnecting the primary side. In that case grounding the secondary side will not do anything for a fault since there is no secondary side OCPD to trip. You can't count on the primary side OCPD to trip on a WYE-Delta transformer with a fault in the secondary side, see NEC 240.21(C)(1).